Yu. A. Chestnov

Total cross sections for fission of heavy nuclei by intermediate-energy protons

The results of measuring total cross sections for proton-induced fission of natPb, 209Bi, 232Th, 233U, 235U, 238U, 237Np, and 239Pu nuclei at proton energies from 200 to 1000 MeV are reported. The measurements were carried out in steps of ≈100 MeV. Complementary fission fragments were detected in coincidence by two parallel-plate avalanche gas counters placed in the beam line on both sides of the thin target close to it. Energy and isotope dependences of the fission cross sections are analyzed.

Inelastic interactions of fast nucleons with nuclei and fission of nuclei

The cross section for the fission of actinide nuclei that is induced by fast neutrons is considered as a fraction of the cross section for inelastic nucleon interaction with nuclei. In turn, inelastic nucleon interaction with a nucleus is treated as scattering on intranuclear nucleons. It is shown that this interaction model describes satisfactorily the cross section for the inelastic interaction of 60- to 2200-MeV nucleons for a broad set of nuclei and that the energy dependence of the cross section for the fission of actinide nuclei that is induced by 400- to 1000–MeV protons replicates the energy dependence of the cross section for inelastic interactions with respective nuclei. From the model used, it follows that the cross sections for proton-nucleus interactions exceed cross sections for respective neutron-nucleus interactions in the energy range extending up to 550 MeV; at higher energies neutron cross sections are larger than proton cross sections.

New approximations to the energy dependences of the total cross sections for the proton-induced fission of 197Au, 203Tl, natPb, and 209Bi nuclei

The total cross sections for 197Au and 203Tl fission induced by protons of energy varied from about 200 to 1000 MeV with a step of about 100 MeV are measured. New approximations to the energy dependences of the cross sections for the proton-induced fission of 197Au, 203Tl, natPb, and 209Bi nuclei are presented and discussed. For all of these nuclei, exponential functions are used as approximations.

Isotopic effect in the energy dependence of the total fission cross section of lead and 209Bi nuclei for protons with energies up to 1 GeV

The results from measurements of the total fission cross sections of 206Pb, 207Pb, and 208Pb nuclei for protons with energies from 207 to 1000 MeV and a step of ≈100 MeV. These results complemented by our data for 209Bi and the data of the EXFOR world base allow determination of absolute and relative contributions of the isotopic effect.

New approximation of the energy dependences of total cross sections for proton-induced fission of actinide nuclei

The energy dependences of the cross sections for the fission of 232Th to 239Pu nuclei that is induced by protons of energy in excess of 50 MeV are proposed to be approximated by the sum of two functions. Of these, the first decreases exponentially with increasing proton energy. It represents the contribution of two-step fission, which competes with particle evaporation. The second function decreases exponentially as the energy grows to about 200 MeV, whereupon it increases up to an energy of 1 or 2.5 GeV. After that, it again decreases for 232Th to 233U nuclei or remains nearly unchanged for 237Np and 239Pu nuclei. This function is likely to represent the sum of the contributions from three modes of single-stage fission.

Quasielastic channel of single-stage nuclear fission induced by 1-GeV protons

Experimental indications that there exists single-stage fission of 238U and 232Th nuclei that is induced by a nearly transverse flux of intranuclear nucleons are obtained from an experiment that employs a two-arm time-of-flight spectrometer exposed to a beam of 1-GeV protons. A nearly transverse flux of mean multiplicity about 10 to 15 nucleons is initiated by a recoil nucleon of mean momentum about 240 MeV/c in the quasielastic scattering of an incident proton.

Experimental Study of Energy Dependence of Proton Induced Fission Cross Sections for Heavy Nuclei in the Energy Range 200–1000 MeV

The results of the total fission cross sections measurements for natPb, 209Bi, 232Th, 233U, 235U, 238U, 237Np and 239Pu nuclei at the energy proton range 200–1000 MeV are presented. Experiments were carried out at 1 GeV synchrocyclotron of Petersburg Nuclear Physics Institute (Gatchina). The measurement method is based on the registration in coincidence of both complementary fission fragments by two gas parallel plate avalanche counters, located at a short distance and opposite sides of investigated target. The insensitivity of parallel plate avalanche counters to neutron and light charged particles allowed us to place the counters together with target immediately in the proton beam providing a large solid angle acceptance for fission fragment registration and reliable identification of fission events. The proton flux on the target to be studied was determined by direct counting of protons by scintillation telescope. The measured energy dependence of the total fission cross sections is presented. Obtained...

Polarization parameters of elastic pp scattering in the angular range 22.5°–42.5° at an energy of 1 GeV and partial-wave analysis of data

The polarization properties of elastic scattering are studied experimentally for pp interaction at 1 GeV. The respective experimental results are presented for the spin-transfer parameter Kn00n and the polarization Pn000 at c.m. angles of 22.5°, 27.5°, 32.5°, and 42.5° and for the analyzing power A000n at c.m. angles of 22.5° and 27.5°. The experiment employed an unpolarized proton beam from the accelerator of the Petersburg Nuclear Physics Institute, a polarized proton target, and a high-resolution magnetic spectrometer. The data from this experiment are analyzed and compared with the predictions of partial-wave analyses.

Experimental observation of a two-phase state of expanding nuclear matter at Ep=1 GeV

A double-arm time-of-flight spectrometer was used to study the thermal multifragmentation of 238U, 232Th, and 197Au nuclei that is induced by 1-GeV protons. It is established that the pseudoevaporation channel, where a single heavy fragment of mean mass in the range 80–90 amu is produced in association with approximately ten smaller clusters (〈M〉=7–10 amu) emitted anisotropically at low velocities, is dominant at this energy. The pseudoevaporation cross sections change from 10±5 mb for 197Au to 120±40 µb for 238U. For 238U and 232Th, it is found that, in the region M<8 amu, the power law is violated, which is explained by the decay of part of small primary clusters in the course nucleon exchange through the “gas” phase within the expanding nucleus.